The invention relates to improving coating devices, in particular, to improving a device for producing coatings by means of ion sputtering of substances in an ionized gas atmosphere. The invention is applicable for producing artificial diamond film coatings, as well as multilayer or sandwich coatings.
The present invention is an improvement of an earlier invention disclosed in U.S. Pat. No. 3,840,451.
According to the previous invention, in a vacuum chamber containing an ionized inert gas there are simultaneously sputtered two graphite cathodes. The sputtering is effected by electric discharge ions. Atoms of carbon that are released in the course of sputtering are deposited on a solid substrate.
There are known different types of devices for coating articles by means of ion sputtering of substances in ionized gas contained in a closed vessel. Such devices employ different sputtering systems, including diode, triode, and tetrode systems. There are also known devices with an autonomous ion source and what is referred to as pinpoint ion sputtering devices.
The diode-type devices are the simplest and cheapest. Such devices comprise a vacuum chamber, wherein there are arranged a cathode and an anode. An article to be coated is mounted on the anode. The device is provided with a vacuum evacuation system, an inert gas supply system, and a power supply system. The vacuum chamber is first evacuated and then filled with an inert gas. Voltage applied between the anode and cathode brings about a gas discharge, whereby a plasma is produced. The plasma consists of electrons, ions, and atoms of the inert gas, as well as of ions, molecules and atoms of residual gases of the vacuum system (N.sub.2 O.sub.2, H.sub.2 O, CO.sub.2, C.sub.n H.sub.m, etc.). Under the action of the electric field, positive ions of the inert and residual gases bombard the cathode. The cathode is sputtered and emits electrons and neutral atoms of the cathode substance whose energy reaches several tens of electron-volts. Neutral atoms of the cathode substance are deposited on the article being coated, which serves as a substrate, in the form of a thin film.
Diode-type devices are disadvantageous in that they must operate at a comparatively high inert gas pressure (about 10.sup.-2 torrs), because at lower pressures such systems either fail to produce a gas discharge or produce an unstable discharge. As a result, neutral atoms of the cathode substance are dispersed and lose their energy on the way from the cathode to the substrate due to numerous collisions with inert and residual gas particles. The resultant coating (film) lacks density and durability and contains impurities. Another disadvantage of such devices resides in the constant bombardment of the freshly deposited coating by the plasma ions and electrons. This, too, affects the coating structure and accounts for lack of density and durability of the coating and a high impurity content therein.
Triode and tetrode devices differ from diode-type devices in that they include a thermionic cathode which is a source of thermoelectrons and is used to forcefully maintain a gas discharge. In a triode system, thermoelectrons are made into a gas discharge plasma with the aid of an anode, whereto a positive potential is applied. The material being sputtered (the target) is mounted on a third electrode whose potential is negative with respect to the plasma. Articles to be coated are secured in a holder opposite the target. After the cathode is hot, and after applying an anode voltage and filling the discharge chamber with an inert gas whose pressure reaches 10.sup.-3 torrs, there is produced a gas discharge. If a negative potential of several hundreds of volts is applied to the target, intensive bombardment of the latter with positive plasma ions starts. The material of the target is sputtered and deposited on the articles being coated.
Tetrode-type devices differ from triode-type devices in the presence of a fourth electrode arranged close to the thermionic cathode, which electrode facilitates ionization of the gas discharge plasma and makes it possible to reduce the inert gas pressure in the course of operation to (2 - 4).times.10.sup.- 4 torrs.
Triode and tetrode devices are disadvantageous in that the freshly deposited film is bombarded with electrons and ions of the plasma. In addition, the presence of the thermionic cathode in such devices makes it impossible to carry out reactive sputtering. Furthermore, the hot cathode is a source of additional impurities in the film.
Devices with an autonomous ion source comprise two chambers, an ionization chamber and a working chamber. The two chambers are separated by a diaphragm having a small orifice. The ionization chamber is filled with an inert gas. The pressure in this chamber may be considerably higher (10.sup.-2 torrs) than that in the working chamber (10.sup.-5 torrs). In the course of operation, in the ionization chamber there are produced charged particles which pass through the orifice in the diaphragm to the working chamber, are focused by an electric and external magnetic fields and bombard a target. The target is sputtered, so there is produced a single source of neutral atoms of the target's substance. Articles being coated are arranged normally with respect to the atomic beam emitted from the target.
Devices with an autonomous ion source provide for reactive sputtering by way of supplying a reactive gas (for example, oxygen or nitrogen) directly to the working chamber, which prevents the destruction of the thermionic cathode of the ionization chamber. This system, however, is by far more sophisticated than all the other systems and must, as a rule, be provided with a thermionic cathode which is a source of additional impurities in the coating. Also, devices with an autonomous ion source can only employ one target for sputtering.
Still another device for producing coatings in the form of thin films is referred to as a pinpoint ion sputtering device. The basic component of such a device is a discharge chamber which comprises a long cylinder-shaped anode. Arranged inside the anode are two cathodes. One of the cathodes is adapted for moving in two directions in the plane perpendicular to the anode axis. Articles to be coated are mounted on the outside of the anode. The coating material reaches the articles through holes made in the anode wall. In order to stabilize the temperature of the articles, they are enveloped by a copper screen. In order to degas the discharge chamber and the articles prior to the sputtering process, the chamber is suspended on a copper arm at whose end there is secured a container for a heater element. In the course of operation, the container is filled with a cooling agent and serves for cooling the discharge chamber. The discharge chamber and the copper screen are contained in a sealed glass cylinder-shaped housing, upon which there is mounted a solenoid to produce a longitudinal magnetic field. A spectroscopically pure inert gas is supplied directly to the discharge chamber. One of the anode orifices may be closed with a metal shutter controlled from the outside, which makes it possible to improve the purity of the coating and accurately measure doses of the sputtered material.
The device under review operates as follows. As the device is energized, between the two cathodes there is produced a gas discharge in the form of an incandescent plasma column. At the point of contact between the column and the movable cathode, the cathode material is sputtered. Through a hole in the anode the sputtered material is passed to the article being coated and is deposited thereon.
The advantages of this device include the absence of a hot cathode and the possibility to still further reduce the inert gas pressure, as compared to tetrode systems. In addition, the article being coated is located outside the discharge zone, which rules out bombardment of the article by plasma ions and electrons and improves the quality of the coating.
That notwithstanding, the device also has a number of disadvantages. The basic disadvantage is the fact that the coating (film) is produced by atoms arriving from only one pinpoint sputtering area, and that atoms are deposited on the article only on the side and, as a rule, at an angle of 90.degree. . This often results in a loose and flimsy coating which has microcracks and micropores. Another disadvantage of the device in question resides in partial penetration into the atomic beam of atoms of residual gases and other impurities, which is due to the following factors:
release of gases from the uncooled cathodes which are heated in the course of sputtering;
release of gases from the anode and other components of the discharge chamber due to incomplete degassing of the device prior to the sputtering operation, and insufficient cooling in the course of operation;
release of gases from packings in the inert gas supply system, and the absence of means for purifying the inert gas before it is supplied to the discharge chamber and
release of gases from rubber seals in immediate proximity to the discharge chamber.
In addition, the pinpoint ion sputtering device is disadvantageous in the it does not have any means for completely removing impurities from the surface of the article being coated (oxides, etc.) prior to the coating operation. The result is insufficient or unsatisfactory adhesion of the coating (film) to the article.
Pinpoint ion sputtering is also disadvantageous in its low efficiency, especially in producing multilayer or sandwich films. Other disadvantages of this type of device include the following:
insufficient purity of the material in a separate layer in producing multilayer coatings (films) and
the absence of substrate heating means, considering that substrates have to be heated when producing monocrystalline or macro-crystalline coatings by means of epitaxy.
It is an object of the present invention to eliminate the above disadvantages.